This invention relates to devices for improving the quality of electrophotographically formed images. In one aspect, the present invention relates more particularly to an electrophotographic apparatus such as a copying machine capable of producing clear images regardless of changes in the ambient temperature. In another aspect, the present invention relates to a device for controlling the temperature of a photoreceptor such as a photosensitive drum used in electrophotography such that clear images can be obtained consistently.
In an electrophotographic image forming apparatus such as a copying machine, the surface of a photoreceptor such as a photosensitive drum is electrostatically charged in single polarity and is exposed to light. An electrostatic latent image is formed on the surface of the photoreceptor and a visible image is produced therefrom by electrostatic adsorption of developing agent. A corona discharger is used for charging the photoreceptor surface but the resultant surface potential sensitively affects the adsorption characteristics of the developing agent and hence the quality of the image transferred onto a transfer medium such as copy paper. Besides, the surface potential of the photoreceptor is easily affected, for example, by changes in ambient conditions such as temperature. Even if the same current is provided to the corona discharger, therefore, the photoreceptor surface potential is not always raised to the same level. In view of the above, it has been considered to provide a heater by means of which the photoreceptor surface can be maintained at a desired temperature level.
The adsorption characteristics of the photoreceptor surface are also affected by the degradation of the surface conditions of the photoreceptor. After a long period of use, for example, the photoreceptor surface may become dirty and a resultant drop in the surface potential tends to cause reduced image density and non-uniform image formation. For this reason, there have been attempts to vary the current supplied to the charger or the bias voltage applied to the developing tank when the image is developed. If the corona current is increased excessively, however, leaks become likely to develop in the photoreceptor and the effects of ozone may become significant and adversely affect the image quality.
In addition to ozone which oxidizes the photoreceptor surface, compounds generated by corona discharge such as NO.sub.x and HNO.sub.3 are generally highly hygroscopic. In a highly humid environment, they absorb moisture in air and gradually degrade the photosensitivity characteristics of the photoreceptor. The photoreceptor therefore becomes incapable of retaining an electrostatic latent image and begins to produce blurry images. A conventional method to prevent this problem has been to provide a heater for the photoreceptor to keep it at a high temperature and thereby prevent it from absorbing moisture. If the photoreceptor, as well as parts and components such as toner and a cleaner blade which are in contact therewith or adjacent thereto, is constantly exposed to a high temperature, however, the image formed thereon becomes dull in the case of a selenium drum because of crystalization and white dots begin to appear in the case of a photoreceptor containing an organic photosensitive material because even small defects become visible in the image.